1. Field of the Invention
The invention relates to a circuit arrangement for an adjustable high-pass filter, comprising a main path for conveying a main current, including a first resistor which is connected at the output to a sum node, a sub-path for conveying a sub-current via a voltage divider unit, a first amplifier stage, whose output is coupled to inverting inputs of the first amplifier stage, and a capacitor which is arranged between the output of the first amplifier stage and the sum node, and a second resistor which is connected to the sum node, with the main current and the sub-current being added at the sum node.
2. Description of the Related Art
In conventional systems of radio receivers or car radios, transmission errors for higher frequencies in the audio baseband are caused by the selectivity of the ceramic filters used. After decoding the signal in the stereo decoder, these errors lead to asymmetrical or poor crosstalk between the right and left channel and are compensated by means of an adjustable high-pass filter. The signals to be decoded are filtered in ceramic filters which, in addition to deviations of their characteristic data, also have a low-pass effect, thus causing an increased non-uniform crosstalk of the two channels of a stereo signal.
The magazine "Electronics" Mar. 10, 1981, pp. 191-192 describes a system in which the filter properties are changed by means of changeable parameters. The signal components passed through the filters are admixed to the main component of the signal after they have been influenced in an amplifier stage, and their variations are added. However, the filters comprise components which cannot be integrated because of their size. Moreover, a DC offset on the basic signal is produced at the mixing point so that a distortion factor distorts the desired signal during the addition. Ground is used as a reference potential.
European Patent Specification EP 0 091 160, corresponding to U.S. Pat. No. 4,509,021, describes a system in which the non-inverting input of an amplifier stage is connected to a voltage divider. The partial voltages are applied to the non-inverting input via a switch. The inverting input is also connected via a further switch to a further voltage divider via which the inverting input is coupled to the output.
A station is filtered from a received frequency band. Ceramic filters are used for this purpose. Frequently, also a plurality of ceramic filters in a series with inter-arranged amplifier stages is connected in series. Ceramic filters have the drawback that they have stray values of their characteristic data. Moreover, the use of ceramic filters evokes a low-pass effect which limits the frequency band of the stereo signal in a non-uniform manner. The transmission errors caused by the selectivity are compensated by means of an adjustable high-pass filter for higher frequency components. After decoding of the signal in the stereo decoder, these transmission errors lead to an asymmetrical or poor crosstalk of the two audio channels. The existing circuits have the drawback that external potentiometers are used for the adjustable high-pass filters so that an extra adjustment must be performed for each circuit or for each apparatus. External potentiometers are not resistant to ageing so that there is a constant change of the transfer function. Hitherto, the integration of large capacitances has not been realized. Ground as reference potential is a drawback because the system ground is beset with disturbances. Moreover, the value of such a reference potential cannot be changed.